Variable displacement pump control

ABSTRACT

A variable displacement pump control having, in addition to a pressure compensator (with or without an auxiliary modulator), a multi-purpose valve which is open at pump startup to purge air therefrom, which is automatically closed after purging of the air, and which is opened in the manner of a relief valve responsive to a sudden rise in pump discharge pressure caused as by rapid closing of a directional control valve. The control is further characterized in that the pressure compensating pilot valve also constitutes a pilot valve for the multi-purpose valve to permit opening of a relief passage by the latter when pump discharge pressure suddenly rises.

BACKGROUND OF THE INVENTION

A variable displacement pump control as for an axial piston pump havinga swash plate actuating piston generally provides for pressurecompensation with or without auxiliary modulation to control the fluidpressure applied to the swash plate actuating piston to maintain apredetermined pressure drop across a variable area metering orificeembodied as in a directional control valve for a fluid motor. Thefollowing U.S. patents exemplify variable displacement pump controlsincluding pressure compensation with or without auxiliary modulation;U.S. Pat. Nos.

3,512,178, May 12, 1970, Russell;

3,726,093, Apr. 10, 1973, Malott;

3,866,419, Feb. 18, 1975, Paul;

3,941,513, Mar. 2, 1976, Malott;

3,945,764, Mar. 23, 1976, Marietta.

Variable displacement pumps of the character indicated are oftenemployed in conjunction with hydraulic systems including one or moredirectional control valves for controlling the operation of acorresponding number of fluid motors, and in existing controls, air maybe introduced into the hydraulic system upon pump startup if there isair in the pump or in the intake line thereof. Moreover, in existingcontrols, the pump may be subject to pressure peaks or spikes of highintensity due to, for example, rapid closing of a directional controlvalve.

In the U.S. Pat. Nos. 3,941,513 and 3,945,764, a constant pressure pilotvalve controls a modulator which bleeds off pressure acting on the swashplate actuating piston, thus to provide the pressure compensationfeature and the modulator, in addition, has opposed areas exposedrespectively to pressures upstream and downstream of a variable areaorifice to maintain a predetermined pressure drop across said variablearea orifice, whereby if the pressure drop exceeds the predeterminedpressure drop, the modulator bleeds off the pressure acting on the swashplate actuating piston to effect decrease in pump displacement. In theevent of malfunction of the modulator, such as sticking thereof, thepump may be overloaded even if a horsepower modulator is provided ascontemplated in said U.S. Pat. Nos. 3,941,513 and 3,945,764.

SUMMARY OF THE INVENTION

A variable displacement pump control having, in addition to a pressurecompensating modulator controlled by a pilot valve (with or without anauxiliary modulator), a normally open valve in communication with thedischarge port of the pump through which air in the pump and its intakeline is purged to the low pressure zone or reservoir upon pump startup,and which is actuated to closed position by buildup of oil pressure inthe pump discharge. Said valve also functions as a relief valve underthe control of the pressure compensating pilot valve to limit pressurepeaks or spikes caused as by sudden blockage of the pump discharge lineby rapid closing of a directional control valve or at the end of thestroke of a fluid motor controlled by the directional control valve.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view showing the variable displacement pump controlherein secured to the pump housing and operatively associated with adirectional control valve and a fluid motor; and

FIGS. 2 and 3 are cross-section views on somewhat enlarged scale takensubstantially along the lines 2--2 and 3--3 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The variable displacement pump 1 herein shown by way of illustrativeexample, may be an axial piston pump such as that shown in Malott U.S.Pat. No. 3,726,093 granted Apr. 10, 1973 in which the displacement isvaried by changing the angular position of the swash plate 2, suchchange in swash plate angle being hydraulically effected by control offluid pressure in the bore 3 in which the swash plate actuating piston 4is axially movable. The pump housing 5 has an intake port 6 leading intothe intake chamber 7 and has an outlet or discharge port 8 from whichfluid is delivered under pressure upon driving of the drive shaft 9which is journaled in the pump housing 5 in well known manner.

Bolted or otherwise secured to the pump housing 5 is the valve housing10 for a valve assembly which provides for pressure compensation andauxiliary modulation of the pump 1, which provides for purging of airfrom the pump 1 upon start-up thereof, and which provides for relief ofexcess oil pressure to limit pressure peaks or spikes or to prevent pumpoverloading in the event of malfunction of the pressure compensating andauxiliary modulator. A suitable gasket 11 seals the joint between thevalve housing 10 and the pump housing 5.

The valve housing 10 has a high pressure passage 12 therethrough, whichat one end communicates with the pump discharge port 8 and which at theother end is connected by the conduit 14 to the inlet port of adirectional control valve 15. The return port of the directional controlvalve 15 is communicated in well-known manner with a suitable tank orreservoir 16. The motor ports of the directional control valve 15 areconnected by conduits 17; 17 to a fluid motor 18 and, as evident, thedirectional control valve 15 is operative to selectively actuate thepiston 19 of the fluid motor 18 in opposite directions. The directionalcontrol valve 15 is preferably of the type which provides a variablearea orifice 20 which varies in size according to the extent of movementof the valve member of the directional control valve 15, thus to controlthe speed of actuation of the fluid motor 18. The fluid pressuredownstream of the variable area orifice 20 is conducted by the conduit21 to the sensing passage 23 in the valve housing 10 and, as hereinafterexplained in detail, an auxiliary modulator 24 in the valve housing 10senses the pressure drop across the orifice 20 and varies the pressurein bore 3 acting on the swash plate actuating piston 4 to achieve theselected flow rate by maintaining a substantially constant pressure dropacross the variable area orifice 20 independently of load pressure ofthe fluid motor 18.

The valve housing 10 is disposed in the pump transmission housing(designated as reservoir 16) and thus the oil return passages 25, 26,and 27 in the housing 10 need not have conduits connected thereto, butsimply open into the transmission housing as shown. The valve housing 10also has a passage 28 which registers with a passage 29 in the pumphousing 5 leading into the bore 3 in which the swash plate actuatingpiston 4 is reciprocable.

Referring now in detail to FIG. 2, the high pressure passage 12 isintersected by a stepped bore 30 which is closed at one end by thefitting 31 and plug 32, the fitting 31 carrying a filter 34 andproviding a passage which intercommunicates the high pressure passage 12with a chamber 35. Slidable in the bore 30 is the auxiliary modulator 24which may be in the form of a valve spool as shown, and which defines achamber 36 to which the low signal pressure from the downstream side ofthe variable area orifice 20 is conducted by conduit 21 and passage 23as previously mentioned.

Screwed into the other end of the bore 30 is a pilot valve body 37,having a seat for a constant pressure pilot valve 38 which is biased bythe spring 39 to seat-engaging position. The biasing force of the spring39 may be adjusted by the adjusting screw 40. Upstream of the seat is anorifice 41. The pilot valve body 37, defines with the valve housing 10 achamber 42 which has communication with the high pressure chamber 35 viathe orifice 43 in the modulator 24. A spring 45 in said chamber 42biases the modulator 24 to the position shown in FIG. 2, the biasingforce of said spring 45 being adjusted to desired value as bysubstituting a washer 46 of different thickness and preferably theconnection of the pilot valve body 37 to the valve housing 10 is suchthat the pilot valve body 37 may be unscrewed as much as say one fullturn, while yet the washer 47 on the groove of the pilot valve body 37may be tightly clamped against the valve housing 10 by the lock nut 48to retain the O-ring 49 in sealed engagement with housing 10 and body37.

The modulator 24 has an orifice 50 which communicates the high pressurechamber 35 by way of passages 28 and 29 with the bore 3 in which theswash plate piston 4 is receiprocable. The modulator 24 also has bleedpassages 51 for bleeding fluid pressure in the bore 3 to exhaust passage26 to decrease the fluid pressure in the bore 3 thus to permit the swashplate actuating piston 4 and swash plate 2 to move in displacementdecreasing direction when the modulator 24 moves against spring 45 toopen the bleed passages 51 to the exhaust passage 26.

When the pilot valve 38 is closed, the fluid pressures in the chambers35 and 42 acting on area A are equalized through the modulator orifice43 whereby the swash plate actuating piston 4 is exposed to fluidpressure through the orifice 50 equal to that in the discharge passage12 and chamber 35 so long as the pressure drop across the variable areaorifice 20 is of predetermined value acting on opposite annular areas ofthe chambers 35 and 36 of the modulator 24.

Should the pressure drop across the variable area orifice 20 increase,due to increased flow above that for which the directional control valve15 has been actuated, the increased differential of pressure acting onthe annular areas of the modulator chambers 35 and 36 will urge themodulator 24 in a direction to compress the spring 45 and to meter fluidfrom bore 3 through the bleed passages 51 thus to decrease the fluidpressure acting on the piston 4 to permit shifting of the swash plate 2to pump displacement decreasing direction until the pressure drop acrossthe variable area orifice 20 reaches a predetermined value such that thespring 45 will urge the modulator 24 to a position closing the bleedpassage 51.

With reference to pressure compensation, when the pressure in thechamber 42 reaches a value sufficient to open the pilot valve 38, thechamber 42 will be communicated with the exhaust passage 27 via theorifice 41 and the open pilot valve 38, and the modulator orifice 43will cause a drop in pressure in the chamber 42 with respect to thepressure in the chamber 35, whereby the modulator 24 will again move tocompress the spring 45 to open the bleed passages 51 to bleed offpressure acting on the piston 4 with consequent movement of swash plate2 in displacement decreasing direction. Should it be desired to havepressure compensation without auxiliary modulation, the passage 23 inthe valve housing 10 may be plugged whereby the modulator 24 will moveto bleed off position responsive to opening of the pilot valve 38.

Referring now to FIG. 3 the valve housing 10 has another bore 55 whichintersects the high pressure discharge passage 12, one end of said bore55 having a seat member 56 therein which provides the exhaust or reliefpassage 25 opening into the high pressure discharge passage 12. Theother end of the bore 55 is closed by the plug 57. Slidably sealed inthe portion of the bore 55 adjacent its intersection with the highpressure discharge passage 12 is a valve member 58 which is biased bythe spring 59 to a normally open position communicating the highpressure discharge passage 12 with the exhaust passage 25. A spacersleeve 60 between the plug 57 and the valve member 58 limits themovement of the valve member 58 in its open direction. The valve memberbore 55 and plug 57 define a chamber 61 which is in communication withthe pilot valve chamber 42 by way of the valve housing passages 62 and63.

As will now be described, the valve member 58 is both an air purgingvalve and a relief valve. When the pump 1 is started if there is air inthe pump or its intake line, the air will be purged through the passages12 and 25, because the pressure differential across the open valvemember 58 is insufficient to compress the spring 59 for movement of thevalve member 58 into engagement with the seat member 56. However, assoon as the air has been purged, the oil flow will cause a pressure dropat the valve member 58 in relation to the pressure which acts in thechamber 61 via the pilot valve chamber 42 and the modulator orifice 43.Moreover, the pressure in chamber 61 acts on a larger area of the valvemember 58 so that once the valve member 58 is in seated position, equalpressures on opposite sides of the valve member 58 will retain it inseated position during normal operation of the pump 1 and the pressurecompensating and auxiliary modulation movements of the modulator 24.Accordingly, the valve member 58 will be retained in seated positionduring increase or decrease of pump capacity by movements of themodulator 24 owing to pressure differentials in the chambers 35 and 36or chambers 35 and 42 responsive to opening and closing of the pilotvalve 38. In normal operation, when the pilot valve 38 opens thepressure drop across the orifice 41 and orifice 43 is such that thepilot chamber pressure 42 will not decrease sufficiently to permit thespring 59 and pressure acting on the smaller annular area of the valvemember 58 to open the valve member 58.

However, if the discharge line 14 is suddenly blocked as by rapidshifting of the directional control valve 15 from operating position toneutral position, or upon the piston 19 reaching the end of its strokewhile the directional control valve 15 remains in operating position,there will be a high pressure peak or spike which will cause opening ofthe pilot valve 38, and the corresponding increased pressure drop acrossthe orifice 43 will reduce the pressure in the chamber 42 and in thevalve chamber 61 so that the high pressure acting on the valve member 58together with the biasing force of the spring 59 will open the valvemember 58 in the manner of a relief valve to communicate the highpressure discharge passage 12 with the exhaust or relief passage 25 thusto limit the pressure peak or spike. Without this relief valve feature,the pump 1 may be operating under extremely high pressure until it hastime to destroke, such destroking time generally being quite long owingto the slow gradual operation of the modulator 24 in normalcircumstances.

Another feature of the valve member 58 is that should the modulator 24malfunction as by sticking in the stepped bore 30, the rising pressurewhile sufficient to open the pilot valve 38 will be continued to beapplied on the swash plate actuating piston 4 through the orifice 50with possibility of overloading of the pump 1. However, in the presentcase, when the pilot valve 38 opens, the pressure drop in the chamber 42will permit opening of the valve member 58 to quickly relieve the highpressure discharge passage 12 whereby the pressure in the bore 3 willbleed through the modulator orifice 50 to permit destroking of the pump1.

In summary therefore, the variable displacement pump control herein hasa dual purpose air purging-relief valve 58 which is operativelyassociated with the pressure compensating and auxiliary modulationfeatures of the control so that said dual purpose valve 58 allowspurging of air from the pump 1 and its intake line upon pump startup andconstitutes a relief valve under the control of the pressurecompensating pilot valve 38 to limit pressure peaks in the pumpdischarge 12 due, as, for example, by rapid closing of a directionalcontrol valve 15 and also as a relief valve should the auxiliarymodulator 24 be malfunctioning.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Control means for avariable displacement hydraulic pump of the type including a housingassembly having pump means, high and low fluid pressure zones, and afluid pressure operated member operatively engaged with said pump meansto vary the displacement thereof, said fluid pressure operated memberhaving restricted fluid communication with said high pressure zone; saidcontrol means comprising a valve member movable in said housing assemblyto open and close a bleed passage from said fluid pressure operatedmember to said low pressure zone and having first opposed areasincluding an area on one side thereof exposed to fluid pressure in saidhigh pressure zone and an area on the other side exposed to fluidpressure in said high pressure zone via orifice means; spring means insaid housing assembly biasing said valve member to bleed passage closingposition; and a pilot valve in said housing assembly opened uponincrease of fluid pressure in said high pressure zone to predeterminedmagnitude to bleed fluid pressure acting on the area on said other sideof said valve member to said low pressure zone whereby predominant fluidpressure upstream of said orifice means acting on the area on said oneside of said valve member is effective to move it to a position to opensaid bleed passage for movement of said fluid pressure operated memberto decrease the capacity of said pump means; and another valve membermovable in said housing assembly responsive to opening of said pilotvalve and to abnormal increase in pressure drop across said orificemeans to open a relief passage from said high pressure zone to said lowpressure zone; said another valve member when in relief passage closingposition having unequal opposed areas of which the smaller area isexposed to said high pressure zone and of which the larger area isexposed to said high pressure zone via said orifice means.
 2. Thecontrol means of claim 1 wherein spring means acting on said anothervalve member assists the fluid pressure acting on said smaller area inmoving said another valve member to open said relief passage asaforesaid.
 3. The control means of claim 2 wherein said relief passageis open during start-up of said pump means to purge air from said pumpmeans and is closed by said another valve member as fluid pressurecommences to build up following such purging of air.
 4. Control meansfor a variable displacement hydraulic pump of the type including ahousing assembly having pump means, high and low fluid pressure zones,and a fluid pressure operated member operatively engaged with said pumpmeans to vary the displacement thereof, said fluid pressure operatedmember having restricted fluid communication with said high pressurezone; said control means comprising a valve member movable in saidhousing assembly to open and close a bleed passage from said fluidpressure operated member to said low pressure zone and having firstopposed areas including an area on one side thereof exposed to fluidpressure in said high pressure zone and an area on the other sideexposed to fluid pressure in said high pressure zone via orifice means;spring means in said housing assembly biasing said valve member to bleedpassage closing position; and a pilot valve in said housing assemblyopened upon increase of fluid pressure in said high pressure zone topredetermined magnitude to bleed fluid pressure acting on the area onsaid other side of said valve member to said low pressure zone wherebypredominant fluid pressure upstream of said orifice means acting on thearea on said one side of said valve member is effective to move it to aposition to open said bleed passage for movement of said fluid pressureoperated member to decrease the capacity of said pump means; and anothervalve member movable in said housing assembly responsive to opening ofsaid pilot valve and to abnormal increase in pressure drop across saidorifice means to open a relief passage from said high pressure zone tosaid low pressure zone; and another orifice means limiting the pressuredrop across said orifice means during normal operation of said valve andpilot valve members so that said another valve member is retained inposition closing said relief passage except when there is an abnormalincrease in pressure drop across said orifice means due to rapid rise offluid pressure in said high pressure zone; said another valve memberhaving opposed areas respectively exposed to said high pressure zone andto said high pressure zone between said orifice means and said anotherorifice means.
 5. The control means of claim 4 wherein the opposed areasof said another valve member are unequal, the smaller area being exposedto said high pressure zone and the larger area being exposed to saidhigh pressure zone between said orifice means and said another orificemeans.
 6. The control means of claim 5 wherein spring means acting onsaid another valve member assists the fluid pressure acting on saidsmaller area in moving said another valve member to open said reliefpassage as aforesaid.
 7. Control means for a variable displacementhydraulic pump of the type including a housing assembly having pumpmeans, high and low fluid pressure zones, and a fluid pressure operatedmember operatively engaged with said pump means to vary the displacementthereof, said fluid pressure operated member having restricted fluidcommunication with said high pressure zone; said control meanscomprising a valve member movable in said housing assembly to open andclose a bleed passage from said fluid pressure operated member to saidlow pressure zone and having first opposed areas including an area onone side thereof exposed to fluid pressure in said high pressure zoneand an area on the other side exposed to fluid pressure in said highpressure zone via orifice means; spring means in said housing assemblybiasing said valve member to bleed passage closing position; and a pilotvalve in said housing assembly opened upon increase of fluid pressure insaid high pressure zone to predetermined magnitude to bleed fluidpressure acting on the area on said other side of said valve member tosaid low pressure zone whereby predominant fluid pressure upstream ofsaid orifice means acting on the area on said one side of said valvemember is effective to move it to a position to open said bleed passagefor movement of said fluid pressure operated member to decrease thecapacity of said pump means; and another valve member movable in saidhousing assembly responsive to opening of said pilot valve and toabnormal increase in pressure drop across said orifice means to open arelief passage from said high pressure zone to said low pressure zone;said housing assembly comprising a pump housing having an intake portadapted to be communicated with a fluid reservoir, a discharge port, andan adjustment port leading to said fluid pressure operated member; and avalve housing secured to said pump housing and provided with dischargeand adjustment passages registering with said discharge and adjustmentports; said valve housing having first and second bores intersectingsaid discharge passage and respectively containing said valve and pilotvalve members and said another valve member and providing suchrestricted communication between said discharge and adjustment passagesand providing openings for reservoir discharge from said bleed andrelief passages and from said pilot valve member.
 8. The control meansof claim 7 wherein said another valve member has opposed areasrespectively exposed to fluid pressure in said discharge passage and tofluid pressure downstream of said orifice means via a valve housingpassage intercommunicating said first and second bores.
 9. The controlmeans of claim 7 wherein said another valve member has unequal opposedareas of which the smaller area is exposed to fluid pressure in saiddischarge passage and of which the larger area is exposed to fluidpressure downstream of said orifice means via a valve housing passageintercommunicating said first and second bores.
 10. The control means ofclaim 9 wherein spring means acting on said another valve member assiststhe pressure acting on said smaller area in moving said another valvemember to open said relief passage as aforesaid.
 11. A control means fora variable displacement hydraulic pump of the type which includes ahousing assembly having a pressure chamber to which fluid pressure fromthe high pressure zone of said pump is conducted by way of a restrictedpassage to act on a pump displacement varying means in said chamber;said control means comprising a pilot operated valve assembly in saidhousing assembly having a pilot valve member which opens and closescommunication between the high and low pressure zones of said pumpresponsive to the fluid pressure in said high pressure zone beinggreater than or less than predetermined magnitude, and a spring biasedmain valve member having first opposed areas respectively exposed tosaid high pressure zone and to said high pressure zone via orificemeans, said main valve member being moved to open a bleed passage fromsaid chamber to said low pressure zone when said pilot valve member isopen and to close said bleed passage when said pilot valve is closed,said bleed passage when open decreasing the fluid pressure in saidchamber for movement of said pump displacement varying means to decreasethe displacement of said pump; and another valve member movable in saidhousing assembly responsive to opening of said pilot valve and toabnormal increase in pressure drop across said orifice means to open arelief passage from said high pressure zone to said low pressure zone;said another valve member having unequal opposed areas of which thesmaller area is exposed to said high pressure zone and of which thelarger area is exposed to said high pressure zone via said orificemeans.
 12. The control means of claim 11 wherein spring means acting onsaid another valve member assists the fluid pressure acting on saidsmaller area in moving said another valve member to open said reliefpassage as aforesaid.
 13. The control means of claim 12 wherein saidanother valve member, upon start-up of said pump, purges air therefromthrough said relief passage until fluid pressure builds up to actuatesaid another valve member to overcome said spring means to close saidrelief passage.
 14. Control means for a variable displacement hydraulicpump of the type including a housing assembly having pump means, highand low fluid pressure zones, and a fluid pressure operated memberoperatively engaged with said pump means to vary the displacementthereof, said fluid pressure operated member having restricted fluidcommunication with said high pressure zone; said control meanscomprising a valve member movable in said housing assembly to open andclose a bleed passage from said fluid pressure operated member to saidlow pressure zone and having first opposed areas including an area onone side thereof exposed to fluid pressure in said high pressure zoneand an area on the other side exposed to fluid pressure in said highpressure zone via orifice means; spring means in said housing assemblybiasing said valve member to bleed passage closing position; and a pilotvalve in said housing assembly opened upon increase of fluid pressure insaid high pressure zone to predetermined magnitude to bleed fluidpressure acting on the area on said other side of said valve member tosaid low pressure zone whereby predominant fluid said one side of saidvalve member is effective to move it to a position to open said bleedpassage for movement of said fluid pressure operated member to decreasethe capacity of said pump means; said valve member having second opposedareas respectively exposed to said high pressure zone and to a pressuresensing passage which is communicated with the downstream side of avariable area orifice in the pressure feed path from said high pressurezone to a fluid motor actuated by fluid under pressure flowing throughsaid variable area orifice from said high pressure zone, said valvemember, while said pilot valve is closed, being moved to bleed passageopening position against said spring means when the fluid pressuredifferential acting on said second opposed areas exceeds a predeterminedvalue; and another valve member movable in said housing assemblyresponsive to opening of said pilot valve and to abnormal increase inpressure drop across said orifice means to open a relief passage fromsaid high pressure zone to said low pressure zone; said another valvemember having unequal opposed areas of which the smaller area is exposedto said high pressure zone and of which the larger area is exposed tosaid high pressure zone via said orifice means.
 15. The control means ofclaim 14 wherein spring means acting on said another valve memberassists the pressure acting on said smaller area in moving said anothervalve member to open said relief passage as aforesaid.
 16. Control meansfor a variable displacement hydraulic pump of the type including ahousing assembly having pump means, high and low fluid pressure zones,and a fluid pressure operatively engaged with said pump means to varythe displacement thereof, said fluid pressure operated member havingrestricted fluid communication with said high pressure zone; saidcontrol means comprising a valve member movable in said housing assemblyto open and close a bleed passage from said fluid pressure operatedmember to said low pressure zone and having first opposed areasincluding an area on one side thereof exposed to fluid pressure in saidhigh pressure zone and an area on the other side exposed to fluidpressure in said high pressure zone via orifice means; spring means insaid housing assembly biasing said valve member to bleed passage closingposition; and a pilot valve in said housing assembly opened uponincrease of fluid pressure in said high pressure zone to predeterminedmagnitude to bleed fluid pressure acting on the area on said other sideof said valve member to said low pressure zone whereby predominant fluidpressure upstream of said orifice means acting on the area on said oneside of said valve member is effective to move it to a position to opensaid bleed passage for movement of said fluid pressure operated memberto decrease the capacity of said pump means; said valve member havingsecond opposed areas respectively exposed to said high pressure zone andto a pressure sensing passage which is communicated with the downstreamside of a variable area orifice in the pressure feed path from said highpressure zone to a fluid motor actuated by fluid under pressure flowingthrough said variable area orifice from said high pressure zone, saidvalve member, while said pilot valve is closed, being moved to bleedpassage opening position against said spring means when the fluidpressure differential acting on said second opposed areas exceeds apredetermined value; and another valve member movable in said housingassembly responsive to opening of said pilot valve and to abnormalincrease in pressure drop across said orifice means to open a reliefpassage from said high pressure zone to said low pressure zone; saidhousing assembly comprising a pump housing having an intake port adaptedto be communicated with a fluid reservoir, a discharge port, and anadjustment port leading to said fluid pressure operated member; and avalve housing secured to said pump housing and provided with dischargeand adjustment passages registering with said discharge and adjustmentports; said valve housing having first and second bores intersectingsaid discharge passage and respectively containing said valve and pilotvalve members and said another valve member and providing suchrestricted communication between said discharge and adjustment passagesand providing a sensing passage opening and openings for reservoirdischarge from said bleed and relief passages and from said pilot valve.17. Control means for a variable displacement hydraulic pump of the typeincluding a housing assembly having pump means, high and low fluidpressure zones, and a fluid pressure operated member operatively engagedwith said pump means to vary the displacement thereof, said fluidpressure operated member having restricted fluid communication with saidhigh pressure zone; said control means comprising a valve member movablein said housing assembly to open and close a bleed passage from saidfluid pressure operated member to said low pressure zone and havingfirst opposed areas including an area on one side thereof exposed tofluid pressure in said high pressure zone and an area on the other sideexposed to fluid pressure in said high pressure zone via orifice means;spring means in said housing assembly biasing said valve member to bleedpassage closing position; and a pilot valve in said housing assemblyopened upon increase of fluid pressure in said high pressure zone topredetermined magnitude to bleed fluid pressure acting on the area onsaid other side of said valve member to said low pressure zone wherebypredominant fluid pressure upstream of said orifice means acting on thearea on said one side of said valve member is effective to move it to aposition to open said bleed passage for movement of said fluid pressureoperated member to decrease the capacity of said pump means; said valvemember having second opposed areas respectively exposed to said highpressure zone and to a pressure sensing passage which is communicatedwith the downstream side of a variable area orifice in the pressure feedpath from said high pressure zone to a fluid motor actuated by fluidunder pressure flowing through said variable area orifice from said highpressure zone, said valve member, while said pilot valve is closed,being moved to bleed passage opening position against said spring meanswhen the fluid pressure differential acting on said second opposed areasexceeds a predetermined value; and another valve member movable in saidhousing assembly responsive to opening of said pilot valve and toabnormal increase in pressure drop across said orifice means to open arelief passage from said high pressure zone to said low pressure zone;said another valve member having opposed areas respectively exposed tosaid high pressure zone and to said high pressure zone via said orificemeans.
 18. Control means for a variable displacement hydraulic pump ofthe type including a housing assembly having pump means, high and lowfluid pressure zones, and a fluid pressure operated member operativelyengaged with said pump means to vary the displacement thereof, saidfluid pressure operated member having restricted fluid communicationwith said high pressure zone; said control means comprising a valvemember movable in said housing assembly to open and close a bleedpassage from said fluid pressure operated member to said low pressurezone and having first opposed areas including an area on one sidethereof exposed to fluid pressure in said high pressure zone and an areaon the other side exposed to fluid pressure in said high pressure zonevia orifice means; spring means in said housing assembly biasing saidvalve member to bleed passage closing position; and a pilot valve insaid housing assembly opened upon increase of fluid pressure in saidhigh pressure zone to predetermined magnitude to bleed fluid pressureacting on the area on said other side of said valve member to said lowpressure zone whereby predominant fluid pressure upstream of saidorifice means acting on the area on said one side of said valve memberis effective to move it to a position to open said bleed passage formovement of said fluid pressure operated member to decrease the capacityof said pump means; said valve member having second opposed areasrespectively exposed to said high pressure zone and to a pressuresensing passage which is communicated with the downstream side of avariable area orifice in the pressure feed path from said high pressurezone to a fluid motor actuated by fluid under pressure flowing throughsaid variable area orifice from said high pressure zone, said valvemember, while said pilot valve is closed, being moved to bleed passageopening position against said spring means when the fluid pressuredifferential acting on said second opposed areas exceeds a predeterminedvalue; and another valve member movable in said housing assemblyresponsive to opening of said pilot valve and to abnormal increase inpressure drop across said orifice means to open a relief passage fromsaid high pressure zone to said low pressure zone; said another valvemember when in relief passage closing position having unequal opposedareas of which the smaller area is exposed to said high pressure zoneand of which the larger area is exposed to said high pressure zone viasaid orifice means.
 19. The control means of claim 18 wherein springmeans acting on said another valve member assists the fluid pressureacting on said smaller area in moving said another valve member to opensaid relief passage as aforesaid.
 20. The control means of claim 19wherein said relief passage is open during start-up of said pump meansto purge air from said pump means and is closed by said another valvemember as fluid pressure commences to build up following such purging ofair.
 21. Control means for a variable displacement hydraulic pump of thetype including a housing assembly having pump means, high and low fluidpressure zones, and a fluid pressure operated member operatively engagedwith said pump means to vary the displacement thereof, said fluidpressure operated member having restricted fluid communication with saidhigh pressure zone; said control means comprising a valve member movablein said housing assembly to open and close a bleed passage from saidfluid pressure operated member to said low pressure zone and havingfirst opposed areas including an area on one side thereof exposed tofluid pressure in said high pressure zone and an area on the other sideexposed to fluid pressure in said high pressure zone via orifice means;spring means in said housing assembly biasing said valve member to bleedpassage closing position; and a pilot valve in said housing assemblyopened upon increase of fluid pressure in said high pressure zone topredetermined magnitude to bleed fluid pressure acting on the area onsaid other side of said valve member to said low pressure zone wherebypredominant fluid pressure upstream of said orifice means acting on thearea on said one side of said valve member is effective to move it to aposition to open said bleed passage for movement of said fluid pressureoperated member to decrease the capacity of said pump means; said valvemember having second opposed areas respectively exposed to said highpressure zone and to a pressure sensing passage which is communicatedwith the downstream side of a variable area orifice in the pressure feedpath from said high pressure zone to a fluid motor actuated by fluidunder pressure flowing through said variable area orifice from said highpressure zone, said valve member, while said pilot valve is closed,being moved to bleed passage opening position against said spring meanswhen the fluid pressure differential acting on said second opposed areasexceeds a predetermined value; and another valve member movable in saidhousing assembly responsive to opening of said pilot valve and toabnormal increase in pressure drop across said orifice means to open arelief passage from said high pressure zone to said low pressure zone;and another orifice means limiting the pressure drop across said orificemeans during normal operation of said valve and pilot valve members sothat said another valve member is retained in position closing saidrelief passage except when there is an abnormal increase in pressuredrop across said orifice means due to rapid rise of fluid pressure insaid high pressure zone.
 22. The control means of claim 21 wherein saidanother valve member has opposed areas respectively exposed to said highpressure zone and to said high pressure zone between said orifice meansand said another orifice means.
 23. The control means of claim 21wherein said another valve member has unequal opposed areas of which thesmaller area is exposed to said high pressure zone and of which thelarger area is exposed to said high pressure zone between said orificemeans and said another orifice means.
 24. The control means of claim 23wherein spring means acting on said another valve member assists thefluid pressure acting on said smaller area in moving said another valvemember to open said relief passage as aforesaid.